1. Field of the Invention
This application relates to a resin liquid for an optical fiber coating layer, and an optical fiber comprising a coating layer made of the resin liquid.
2. Description of the Related Art
An optical fiber comprises a naked optical fiber made by forming a strand, a first coating layer formed on the naked optical fiber, and a second coating layer formed on the first coating layer. As resins for the first and second coating layers, in view of productivity, such as cure rate, ultraviolet curable resins, such as urethane acrylates, and epoxy acrylates, are mainly used. In addition, silicone resins, denatured silicone resins and the like are also used.
An optical fiber comprising a coating layer made of ultraviolet curable resins is manufactured by coating an ultraviolet curable resin liquid before curing onto a naked optical fiber; irradiating ultraviolet light and curing the ultraviolet curable resin liquid on the naked optical fiber, thereby preparing a first coating layer which is relatively soft; coating an ultraviolet curable resin liquid before curing onto the first coating layer; and irradiating ultraviolet light and curing the ultraviolet curable resin liquid on the first coating layer, and thereby a second coating layer which is relatively hard is prepared.
There is a possibility that many kinds of extraneous materials will contaminate the ultraviolet curable resin liquid from manufacturing devices, a container thereof, and the like, during manufacturing and transferring. Among extraneous materials, the relatively large extraneous materials having a size more than 1 mm or greater can be removed from the ultraviolet curable resin liquid by filtration during manufacturing. However, it is difficult for the relative small extraneous materials having a size 100 μm or smaller, for example, to be detected visually. Therefore, there is a possibility that small extraneous pieces of material will remain in the resin liquid and be coated onto the naked optical fiber or the coating layer, and thereby the small extraneous materials contaminate in the coating layer.
Long-period transmission characteristics of the optical fiber contaminated with the small extraneous materials in the coating layer sometimes degrade, due to the extraneous materials. Specifically, when heat-cycle tests are carried out repeatedly to test long-period transmission characteristics, optical fibers, in which long-period transmission characteristics decrease whenever heat-cycle tests are carried out, are sometimes found. It was confirmed by observation of the coating layer of the optical fiber in which long-period transmission characteristics degrade that fine air gaps are generated in the coating layer, and that the air gaps are generated from the extraneous materials as starting points. When the extraneous material is harder than the ultraviolet cured resin around the extraneous material, the interface between the ultraviolet cured resin and the extraneous material is peeled due to compressive stress generated by repeated heat-cycle tests, and the peeled portion causes brittle fracture in the ultraviolet cured resin.
Due to this brittle fracture, the size of the air gap increases, and compressive stress due to the air gap causes nonuniform microbends. It is believed that as a result, transmission characteristics of the optical fiber degrades by the nonuniform microbends.
Moreover, the peeling between the ultraviolet cured resin and the extraneous material is generated depending on the number of the extraneous materials in the coating layer. Therefore, it is thought that when the number of the extraneous materials is small, degradation of transmission characteristics hardly occurs.